Coating compositions are well-known in the art and are used in a great variety of applications such as for primers, base coats and clear coats in vehicle original coatings and vehicle repair coatings. In this regard, great efforts have been made to develop coating compositions imparting the desired properties to the substrate or article to be coated. For example, coatings have been developed for e.g. protecting against abrasion, chemicals, corrosion, heat or mechanical impact.
In this regard, aspartate based coating compositions are well known in the art. For example, EP 0 403 921 describes coating compositions with binders based on a polyisocyanate component and an isocyanate-reactive component containing specific secondary polyamines. These secondary polyamines are also called polyaspartic acid derivatives and are based on primary polyamines and diesters of maleic or fumaric acid. EP 0 470 461 also describes coating compositions for vehicle refinish applications containing a polyisocyanate component and an isocyanate-reactive secondary diamine prepared from 3,3′-dimethyl 4,4′-diamino dicyclohexylmethane and maleic diethylester. The isocyanate-reactive component further contains a hydroxyl-functional component consisting of polyhydroxypolyacrylates or mixtures of polyhydroxypolyacrylates and polyesterpolyols. WO 2005/073188 refers to aspartates which are prepared by first reacting a di- or polyamine with an unsaturated ester and then reacting the resultant product with a maleimide. U.S. Pat. No. 5,633,336 A refers to low-viscosity (cyclo)-aliphatic polyamines containing urea groups and having more than two amino groups that may be prepared by reacting (cyclo)aliphatic diamines with polyisocyanates containing isocyanurate groups or biuret groups. Such polyamines are useful as components in PUR reaction finishes and coating and adhesive compositions. WO 2013/098186 A1 refers to an aqueous urethane resin composition consisting mainly of the following (A) component and containing the following (B) and (C) components: (A) hydrophilic polyol, (B) water dispersible polyisocyanate, (C) aspartic acid ester having a secondary amino group. WO 2009/086026 discloses a transparent organic solvent-based clear coat coating composition comprising at least one binder with functional groups containing active hydrogen, in particular hydroxyl groups, at least one polyisocyanate cross-linking agent with free isocyanate groups and at least one epoxy-functional silane. U.S. Pat. No. 4,677,154 discloses a process for improving the color stability of polyurethanes, wherein the polyurethane reaction is conducted in the presence of an additive stabilizer comprising 2,6-di-(t-butyl)-p-cresol (BHT) and a compound selected from specific phosphites, diphosphonites, isocyanurates and other compounds such as di-stearyl thiodipropionate, di-stearyl pentaerythritol and 4,4′-methylene bis(2,6-di-t-butylphenol. The use of the additive package prevents discoloration of the thermoplastic polyurethane caused by discoloration of the polyisocyanate. WO 2004/063242 discloses coating compositions containing aspartic compounds and polyisocyanate curing agents. The coating compositions contain a di-subtituted phenol antioxidant or a hydroperoxide decomposer. Presence of the antioxidant or decomposer leads to coatings with good resistance to UV light
A general disadvantage of the above aspartate based coating compositions if they are combined with UV absorber is that the component containing the curable aspartate remarkably discolours during storage. Accordingly, also the coating composition comprising the curable aspartate tends to discolour during storage such that e.g. the final colour of the obtained coating does not match the desired or targeted colour anymore. However, it has not hitherto been possible to provide a satisfactory solution to avoid this color instability of aspartate based coating compositions on storage. This colour instability of the coating composition is clearly not desired.
Thus, the object of the present invention is to provide a coating composition which does not show discoloration and yellowing on storage at room temperature as well as at elevated temperatures, e.g., of up to 50° C. The coating composition shall be color stable for a time period of at least 12 months, while at the same time the mechanical and optical properties, such as adhesion and interlayer adhesion in a multi-layer structure, abrasion, chemical and corrosion resistance, of the obtained coating are maintained on a high level.
In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with this background.